1. Field of the Invention
This invention relates to an interface circuit for a private branch exchange, a key telephone system, or others which has a function of battery feed to a central office in a telephone network.
2. Description of the Related Art
In telecommunications, electric power is consumed by the transmission of an address signal from a telephone set to a central office, the generation of a ringing tone in the telephone set, and the voice communication between the telephone set and the central office. Generally, such electric power is derived from a dc power source or a battery located in the central office which is referred to as a common battery. Thus, in this case, a battery current for the voice communication is fed from the central office toward the telephone set.
According to special standards for telephone networks, a private branch exchange or a key telephone system is required to have a function of feeding a battery current for voice communication toward a central office. Therefore, the private branch exchange or the key telephone system has an interface circuit for implementing the feed of a voice communication current toward the central office. Such an interface circuit of a prior-art type tends to be affected by common-mode noise.